USB - I have moved images/data from my USB Flash Drive into my “trash can” on my Mac; however, the device is still showing

Even though you have moved the images/ data to the trash they will remain in a trash folder within the flash drive until you use the 'empty trash' option whilst the device is connected to the 'Mac'. Once you have emptied the trash the space will be available again on the flash drive.

This could be the reason why a USB Flash Drive may show full when inserted into a Mac computer.